08 Prostar Cummins ISX load shedding

Something is tweaking out.

Try cleaning the power and grounds off on the batteries and then see what happens. Start small.

 

Mine does it about 30 seconds after I shut the truck off. It’s showing no voltage.
I should give the whole story. Recently I parked the truck to work driving another guy’s for hourly rate rather than wear mine out for crap rates. being far from home in the evening I used mine for a place to watch tv and sleep. Over a 2-3 week period it started off fine. I have the electric (battery) APU with 4 new $350 ea AGM batteries I bought in July 2923 so they were not even 2 months old when I started. I 4 under the step were all new Jan 2023 but they are AGR $80 batteries 925 cca
I’d run off the air off the APU until I needed to charge them up. Then it got to be that even after it had cooled enough to sleep without air the truck barely started in the morning and evening so I’d let it run long periods. Even if the last thing I did was run it long periods then go to sleep or go to work with nothing on it got to the point I had to jump start it with my booster. Every time I’d start it the voltage was up around 14.
A couple days ago I took all the older $80 batteries out, gave them each se real hours to charge at 49 amp setting, all set on a block of wood, and the load tested them. I’d have bet money one was bad. Nope, all 4 good. I took out one from the APU and did the same and would have taken the other 3 out eventually to ensure all of them were good or find a bad one. But first, since the 4 up front under the step were good I put them back in. I started it with the one APU battery still in a charge in the shop. First thing I noticed was it didn’t whip over really fast but acceptable. A couple hrs later I put the APU battery back in and out a charger on those 4 at 49 amp. I started the truck and it whipped over quick. Then I noticed the voltage output in the gauge was showing a little over 12, less than 13. I asked s mechanic that works in this farm his thoughts about the difference specifically could that be because the batteries are all charged? Does it indicate alternator output or voltage required based on demand. Neither of us was sure. I have an inventor wired direct to the APU harness that goes to my original inverter that quit a long time ago and it’s independent of the body control modules commands. It’s been that way over a year so I was able to relax watch tv and cool enough I did t need the air conditioning. Latex I started the truck, whipped over quick and saw the voltage out out quite a bit lower. It’s progressively gotten close to 0. I knew I had problems but before I did anything I wanted to see if the batteries still draw down while sleeping or gone at work. No. It whipped over this morning I went yo work 11 hrs and it whipped over this evening. So whatever was causing those 8 batteries 2-8 months old each to draw diwn enough to require s jump start no longer does that after pulling the batteries out and testing them and giving s hood charge before. But the alternator voltage output that showed 14 before started at 12 and steadily decreased to nothing or close to it.